Glass composition



United States Patent OffiC 2,901,365 Patented Aug. 25, 1959 GLASS COMPOSITION Samuel L. Seymour, Oakmont, and James E. Duncan, Brackenridge, Pa., assignors to Pittsburgh Plate Glass Company, Allegheny County, Pa., a corporation of Pennsylvania No Drawing. Application November 25, 1957 Serial No. 698,328

6 Claims. (Cl. 106--52) The present invention relates to a family of glass compositions having a density below 2.7 grams per cubic centimeter, preferably between 2.55 and 2.65 grams per cubic centimeter and having a refractive index, N of 1.56 to 1.58. Such glasses are desired so that higher magnifications can be obtained in lenses with less glass weight.

Presently made ophthalmic glasses having a relatively high refractive index require the addition of oxides of relatively high densities, such as PhD and BaO to obtain this desired property. The inclusion of such heavy oxides has resulted heretofore in the necessity for increasing the density of lenses provided with a high refractive index to above 3 grams per cubic centimeter. This additional weight is a source of considerable annoyance to wearers of spectacles containing such heavy lenses, especially those people requiring thick lenses. When low refractive index glasses. are used for ophthalmic purposes, lenses of considerable thickness are required to supply the needed magnification. High index glasses required for equivalent magnifications are thinner and more desirable for style considerations.

Accordingly, it is an object of the present invention to provide ophthalmic glasses having lighter weight and a relatively higher index of refraction than has hitherto been available for ophthalmic lenses.

Another object of this invention is to provide ophthalmic glasses for lenses which will give greater comfort to the wearer of spectacles where high magnification is a requirement. It is also an object of this invention to provide ophthalmic glasses which will permit the use of thin lenses for high magnification and thus improve the appearance of spectacles.

A further object of the present invention is to provide a crown type ophthalmic glass for use as the sole portion of spectacle lenses having an index of refraction, N between about 1.56 and 1.58 and a density of not more than 2.7 grams per cubic centimeter, preferably between 2.55 and 2.65 grams per cubic centimeter.

It has been found that the objects of the present invention can be obtained by utilizing glasses having compositions within the ranges set forth below. Table I discloses ranges of proportions of various ingredients that are suitable for providing glasses having the desired characteristics.

The glasses of the invention may be produced from conventional glass making batch ingredients. These batch materials include sand, soda ash, potassium carbonate, lithium carbonate, calcium carbonate, magnesium carbonate, titanium dioxide, boric acid, aluminum hydrate, antimony oxide and arsenious oxide. The batch ingredients are thoroughly mixed together and may be heated under oxidizing conditions in a refractory pot in a furnace by the controlled combustion of natural gas. Various size pots may be employed and the melting temperatures and times will vary according to the amount of glass being formed.

The following temperatures and melting conditions may be employed to melt ten pounds of the glasses of the present invention in a small pot. The empty pot is preheated in a furnace at a temperature of 2200 F. A portion of the thoroughly mixed batch is ladled into the preheated pot and the temperature of the furnace gradually increased. Over a period of 2% hours the remaining portion of the mixed batch is ladled into the pot and the temperature is increased to about 2650 F. to melt all materials. The furnace temperature is maintained at about 2650 F. for an additional 1 /2 hours to permit refining of the glass.

After the glass is refined, the temperature of the furnace is reduced slowly over a 1 /2 hour period to about 2100 F. The pot is removed from the furnace and the glass is poured on a metal table and rolled with a heavy roller into the form of a sheet. The sheet is placed in a kiln and cooled from about 1050 F. to about 850 F. at a rate of 3 F. per minute. Thereafter, it is cooled more rapidly to room temperature and subsequently cut into small pieces suitable for fabrication into lenses. It is to be understood that smaller or larger pots may be employed to form smaller or larger quantities of the glasses. In these cases, slightly different temperatures and time intervals may be employed, for example, the length of time of melting will be longer as larger pots are employed to melt larger quantities of the glasses.

' The combined function of the various constituents in these glasses is to contribute the lowest possible density and the highest possible refractive index to the glasses. A brief description of the contribution of each ingredient to the glasses is now presented.

SiO is the principal glass former. Since it contributes to low density, but not to a high index of refraction, the

amount used must be governed by these contributions.

If more than 64 percent by weight of SiO is present, it becomes difiicult to adjust the proportion of the other ingredients to provide an easily meltable glass having the desired density and refractive index. Similarly, it is difiicult to obtain the desired density in a chemically durable glass having a refractive index over 1.56 when the Si0 content is reduced to below 58 percent by weight of the glass.

The alkali metal oxides are included as fluxes to assist in the melting of the batch ingredients used to form the glasses. Less than 12.5 percent by weight of alkali metal oxides results in a batch diflicult to melt; whereas, more than 17.5 percent by weight results in glasses which are chemically less durable. Na O is most desirable as the alkali metal oxide because it contributes less weight than K 0; however, K 0 in an amount up to 15.5 percent by weight may be employed. Li O is a desirable constituent but is more expensive than N21 O or K 0. It contributes to low weight and a high index of refraction and also improves the batch meltability.

Calcium oxide is a flux and contributes to the chemical durability of the glasses. Since calcium oxide imparts a higher refractive index to the glass than magnesium oxide, its use is preferred even though the latter contributes to a slightly lighter glass than the C210 and small amounts may be present. The range' of (Sat) or GaO plus MgO should be. about .5 to 1.4 Dement- In the glasses of our invention, Ti is the principal constituent contributing low density and high refractive inde c ara s t the glasses. Les than. p rce t y w gh T Oz d es not p eside suflie t e fec in med tyin o h t e. i y and r frac iv de to th desired amount; whereas, over 9 percent by weight of TiO- tends to i pa a o j onabl ye o i c l 0 t e g asse It is also possible to include B 0 in amounts up to percent by weight. Greater amounts of B 0 are not desired because it harmfully reduces the chemical durability of the glasses. Boric oxide imparts low density. It contributes a higher refractive index to the glasses when substituted for SiO It is therefore desirable in small amounts for these reasons.

Similarly, small amounts of A1 0 may also he in; cluded in the glasses since this ingredient has a mildly beneficial effect on both the density and refractive index as desired by the objects of the present invention. Up o 3 per y ei o A 2 5 y be inc uded- If mo th n 3 p r nt o A12 s nc u d. owe t is more difiicult to melt the batch. It is preferred to omit this constituent ordinarily because the other in: gredients recited above have a greater beneficial effect on the desired characteristics Without introducing the melt: ing problems to the degree found with alumina.

In preparing the glasses of the present invention, up to .2 Per n y w g of a re n n a t such a bZQE or A5 0 or combinations thereof may be added to prove the refining of the glasses.

The combination of Ti0 with CaO in an aikali silica glass provides a family of novel glasses suitable for ophthalmic purposes wherein a high refractive index and a low density are desired. These characteristics may be lost in part or completely when the various constituents are not properly balanced within the framework of the preferred compositions recited. A number of glass compositions which are exemplary of the invention are listed in Table II.

Table II Parts By Weight C ns u .4

No.1 No. 2 No.3 No. 4 No.5

61. 6 59. 6 5s. a s9. 6 8.9 6.4 5.9 6.4 6.7 6.7 6.7 6.7 12.6 7.6 12.6 7.6 5.5 9.0 5.5 9.0 4.0 10.0 10.0 8.0 2.0 .7 .7 .'7 .7 .7 1. 5600 1. 5725 1. 5648 1. 5705 V 7 48. 52.0 50.3 53.2 49. 7

Softening point,F 1,336 1,316 1,325 1,335 1,360 Densit arm or cubic centii net r 2.64 2.62 2.60 2.62 2

It is understood that the specific examples recited above by Way of llustration a d o lim at on. he breadth of the present invention is limited by the scope of the accompanying claims. The use of the terminology consisting essentially o in the claims is not intended to exclude the addition of small amounts of other glass forming ingredients which do not adversely eifect the density, refractive index and other optical properties desired. In this regard, small amounts of eolgrants or other materials which may effect the transmission, ab e pt or h r p p t es of he l s m be pre ent- The glasses are useful as lenses in ophthalmic speetacles. As such, they are ground and polished according to prescription by conventional lens grinding and polishing techniques and fitted into spectacle frames.

This application is a continuation-impart of our copending application Serial No. 410,414, filed February 15, 1954, now abandoned.

We claim:

1.- as a in a de i y of -55 to .5 rams Per cub c een m t and a refractive ndex. Na. o 1.5619 1- si i e sen ial y o he f llo n hsredi hts in p rce t y we ht: 58 t9 6 pe ent z. 0 t 17-. Pe e n M20. 0 t9 5- pe cen K29. 0 9 5 Pereeat Li O, the sum total of alkali metal oxides being 12.5 to pe ent. -5 o 4 peree it of lkali earth me o de selected from he roup cons sti of and C t) Pl M0. 5 o 9 p rcen TiQz. Q o 0 Perc nt B203 nd 0 t6 3 Per en A1203. t above, is ed esse tial ing ed en s plus small am un o eelerahts eehstitt ias 00 pe en 'by e ht f h ass 2- A gless having t e f ll in prox mate e mpcsitie in p cen by' ht 36 Pe cent SiOz. pereeht M20. 3 percen K2 12.5 p rcent (Fa 8.2 pereen TiQz and 0.7 percent Sb- O A A l s a in t e fol o n a p x ma e s mesttie in pe e t by e ht: 61-6 P rc n 2. 8- pereent M20. 6- pe cen K20. 2 6 e cen C 5-5 verses! Qz. pe c 203 a Peree t 205:

4. A glass having the following approximate compositions in percent by weight: 59.6 percent SiO 6.4 percent M20. 6- pe K2 -6 per ent Q 0. .0 ere ht T1 2. 1 p e B203 a 0- pe ce t zQs- 5. A glass having the following approximate composi: tions in percent by weight: 5 8.6 percent SiO 5.9 percent aZ pe 2 2-6 P QehtCaQ; 5.5 Pe cent T z. 0. pe e n B203 and 0.7 pe e 205- A glass a ng e o l in pr ma e eempe r tions i pe ent by weight: perc n S Qz. 6- Per ent M20. 6. p c K2 7-6 Pe ce t (3 0. .0 P c nt 2. percent 2 3. p n A an .7 PIl1l zQs- References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,901,365 August 25, 1959 Samuel L. Seymour et al.

It is hereby certified that error appears in the -printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 26, for elkexlf' read alkaline Signed and sealed this 14th day of June 1960.

(SEAL) Attest:

KARL Ho AXLINE ROBERT (J. WATSON Commissioner of Patents Attesting Oificer 

1. A GLASS HAVING A DENSITY OF 2.55 TO 2.65 GRAMS PER CUBIC CENTIMETER AND A REFRACTIVE INDEX, ND, OF 1.56 TO 1.58 CONSISTING ESSENTIALLY OF ATHE FOLLOWING INGREDIENTS IN PERCENT BY WEIGHT: 58 TO 64 PERCENT SIO2, 0 TO 17.5 PERCENT NA2O, 0 TO 15.5 PERCENT K2O, 0 TO 5 PERCENT LI2O, THE SUM TOTAL OF ALKALI METAL OXIDES BEING 12.5 TO 17.5 PERCENT, 7.5 TO 14 PERCENT OF ALKALI EARTH METAL OXIDES SELECTED FROM THE GROUP CONSISTING OF CAO AND CAO PLUS MGO, 5 TO 9 PERCENT TIO2, 0 TO 10 PERCENT B2O3 AND 0 TO 3 PERCENT AL2O3, THE ABOVE LISTED ESSENTIAL IMGREDIENTS PLUS SMALL AMOUNTS OF COLORANTS CONSTITUTING 100 PERCENT BY WEIGHT OF THE GLASS. 